Abstract: Cotton plant nitrogen diagnosis and nitrogen fertilizer recommendation adapting nitrate concentration in the 4th(expanding) petiole as index was studied under field conditions.Reflectance Meter was used to measure nitrate concentration in petiole.The experiment was situated in Fenshou farm,Awati county,Xinjiang.Five fertilization rates(N 0,180,240,300,360 kg/ha) were set up fertilized with urea.The results showed that NO~-_3 concentration in the 4th (expanding) petiole at flowering stage;flowering and boll stage and boll stage was significantly correlated with rates of N applied.Using quadratic model simulating the relation between N fertilization and yield found that optimal economic N application rate was 304 kg/ha with the corresponding yield being 2420 kg/ha.The diagnostic critical value of NO~-_3 concentration were preliminarily set up as 10463 mg/L and 6901mg/L separately for flowing and boll stage;NO~-_3 concentration of cotton in primary growth stages were significantly correlated with lint cotton yield.Based on this phenomenon a cotton nitrogen fertilizer recommendation model was developed and the rate of nitrogen topdressing could be recommended consequently using this model.